Congestive Heart Failure (CHF) is the only form of heart disease still increasing in frequency. According to the American Heart Association, CHF is the “Disease of the Next Millennium”. The number of patients with CHF is expected to grow even more significantly as an increasing number of the “Baby Boomers” reach 50 years of age.
CHF is a condition that occurs when the heart becomes damaged and reduces blood flow to the organs of the body. If blood flow decreases sufficiently, kidney function becomes impaired and results in fluid retention, abnormal hormone secretions and increased constriction of blood vessels. These results increase the workload of the heart and further decrease the heart's pumping ability and, that in turn, causes further reductions in blood flow to the kidney. It is believed that the progressively-decreasing perfusion of the kidney is the principal non-cardiac cause perpetuating the downward spiral of the “Vicious Cycle of CHF”. Moreover, the fluid overload and associated clinical symptoms resulting from these physiologic changes are the predominant cause for excessive hospital admissions, terrible quality of life and overwhelming costs to the health care system due to CHF.
While many different diseases may initially damage the heart, once present, CHF is split into two types: Chronic CHF and Acute (or Decompensated-Chronic) CHF. Chronic Congestive Heart Failure is a longer term, slowly progressive, degenerative disease. Over years, chronic congestive heart failure leads to cardiac insufficiency. Chronic CHF is clinically categorized by the patient's ability to exercise or perform normal activities of daily living (such as defined by the New York Heart Association Functional Class). Chronic CHF patients are usually managed on an outpatient basis, typically with drugs.
Chronic CHF patients may experience an abrupt, severe deterioration in heart function, termed Acute Congestive Heart Failure, resulting in the inability of the heart to maintain sufficient blood flow and pressure to keep vital organs of the body alive. These acute CHF deteriorations can occur when extra stress (such as an infection or excessive fluid overload) significantly increases the workload on the heart in a stable chronic CHF patient. In contrast to the stepwise downward progression of chronic CHF, a patient suffering acute CHF may deteriorate from even the earliest stages of CHF to severe hemodynamic collapse. In addition, Acute CHF can occur within hours or days following an Acute Myocardial Infarction (AMI), which is a sudden, irreversible injury to the heart muscle, commonly referred to as a heart attack.
A. Treatment Strategies for CHF
1. The Treatment of Chronic CHF
There are currently two broad categories for the treatment of Chronic CHF: (1) drug therapy and (2) surgical therapy. All treatments share the common goal of the alleviation of CHF symptoms, the improvement of heart function, and the disruption of the neurohormonal secretions of a kidney to decrease stress and prevent possible failure.
A cornerstone of the drug therapy of Chronic CHF includes the use of angiotensin converting enzyme (ACE) inhibitors, positive inotropic agents, diuretics, digitalis, and, more recently, beta-blockers with the amount of each drug used dependent on the stage of heart failure.
Positive Inotropic Agents
Directly combating the inability of the heart to propel blood forward might seem to be the single most intuitive means for treating heart failure. A class of drugs known as inotropes increases the strength of contraction of the remaining viable heart muscle, allowing the heart to expel more blood with each beat. While all types of inotropes (e.g., dobutamine, dopamine, milrinone) are effective in the short-term, they lack long-term value in the treatment of heart failure because they, like the vasodilators, tend to cause additional neurohormonal activation (as evidenced by hormonal kidney secretions) and perpetuation of the downward spiral.
Diuretics
Diuretics decrease the sodium and water retention in a patient by preventing reabsorption of these substances at specific sites in the renal tubules of the kidney. Diuretics, such as Lasix and Bumex, are effective at reducing symptoms of heart failure due to fluid overload, especially in the lungs and extremities. In the long-term, diuretic therapy fails because it further activates the renin-angiotensin system (e.g., the hormones secreted by the kidney) and eventually overwhelms the ability of diuretics to control salt and water retention.
Vasodilators
The next logical step in the treatment of heart failure is to limit vasoconstriction and reduce its adverse effect on the heart. Unfortunately, vasodilatory agents, like diuretics, fail after a period of time, as they decrease kidney perfusion pressure and activate the renin-angiotensin system.
ACE Inhibitors and β-Blocking Drugs
In the past two decades, the development of angiotensin converting enzyme (ACE) inhibitors and β-blockers has signaled perhaps the most significant development of this century in the pharmacological treatment of heart failure. Both are aimed at the neurohormonal axis of this disease and both act by disruption of the feedback loops that characterize heart failure. β-blockers and ACE inhibitors are the first classes of drugs to be associated with a survival benefit for patients in heart failure. However, despite these significant advances in medical therapy, their effectiveness is limited, especially in the later stages of CHF. Patients become resistive to the increased dose and potency of drugs until further increase becomes too dangerous.
Surgical Therapies
There are three potential surgical treatments for patients in heart failure: (1) revascularization, (2) implantation of a heart assist device, and (3) heart transplantation. Revascularization is the restoration of blood flow to the heart itself, either angiographically (PTCA) or surgically (CABG). Revascularization is performed in patients in whom it is believed that a poor blood supply to the heart itself is the major cause of the observed heart dysfunction. A second surgical modality is the placement of an implantable pump that replaces the failed ventricle. This type of device is known as the Left Ventricular Assist Device (LVAD). The third and ultimate surgical modality for patients suffering from heart failure is transplantation. While this can be an effective means to cure heart failure, transplantation is replete with significant medical issues. In addition, for a majority of patients suffering from CHF, transplantation is not available because they fail to meet current criteria for heart transplant recipients, socioeconomic issues and, most commonly, lack of donor organs. Despite their benefit, surgical therapies are used in less than 1% of all heart failure patients due to their cost, invasiveness and a lack of donors' hearts.
2. Treatment of Acute CHF
Pharmacologic Therapies
In contrast to the treatment of Chronic CHF, the abruptness and severity of the decrease in blood flow and pressure put the vital organs, e.g., the kidneys, at immediate risk of severe damage. Interestingly, the physiologic effects of some therapies used to treat Acute CHF closely parallel those of the “Vicious Cycle” of CHF in that they may transiently or permanently damage some organs to preserve the heart and the brain.
The first line of therapy for Acute CHF is the use of intravenous (IV) inotropic (“squeeze” enhancing) agents in concert with intensive diuretic therapy. The purpose of this therapy is to substantially increase the output of the heart, increasing kidney blood flow, and thereby increasing urine output. It may take hours to days for this therapy to restore hemodynamic stability and fluid removal. In addition, therapy with IV inotropic agents has side effects. Most inotropic drugs can cause vasodilation and hypotension (low blood pressure) that may lead to Acute Renal Failure. Cardiac arrest from inotropic-induced arrhythmias (irregular heartbeats) also can occur.
The second-line therapy is the use of vasopressor (vasoconstricting) agents. While they increase blood pressure, in the higher doses used in Acute CHF, these agents cause severe vasocontriction, and can lead to kidney and liver failure. In concert with a large mandatory fluid intake from multiple IV medications, progressively increasing diuretic unresponsiveness and concurrent hemodynamic instability, reduced renal perfusion leads to the refractory (drug resistive) fluid overload state seen in Acute CHF. This excess fluid decreases ventricular function, oxygenation and other organ function, and impairs the ability to give such additional therapies as increased IV pharmacologic therapy (such as vasopressors) or parenteral nutrition.
3. Mechanical Fluid Removal Therapies
Once pharmacological therapy is exhausted, Continuous Renal Replacement Therapy (CRRT) has been used to treat patients suffering from excess fluid overload. CRRT has been performed previously using standard methods of hemodialysis and continuous arterio-venous hemofiltration (CAVH). More recently, continuous veno-venous hemofiltration (CVVH) has been used to reduce the complications associated with such issues as hemodynamic instability and need for arterial access.
In cases where drug therapy is no longer sufficient to support the patient, effective, intra-aortic balloon pumps (IABPs) are commonly used. IABPs provide limited support of blood flow and pressure in CHF. Other devices, including ventricular assist devices are invasive and costly, but effective at increasing blood flow. Implantation of these devices generally requires the patient to undergo heart transplantation.
4. Failure of the Current Treatments for CHF
Current treatments for CHF share the common goal of the alleviation of symptoms, and the improvement of heart and kidney function. The cornerstone of the medical therapy of chronic CHF includes the use of angiotensin converting enzyme (ACE) inhibitors, positive inotropic agents, diuretics, digitalis, and more recently, beta-blockers with the amount of each drug used dependent on the stage of heart failure. While drug therapy is effective in the early stages of CHF, there is no truly effective drug treatment for the later stages of CHF. Acute CHF is generally treated with intravenous inotropic (“squeeze” enhancing) and vasopressor (blood pressure raising) agents in concert with intensive diuretic therapy. If this therapy fails, the patient can quickly develop severe fluid overload and suffer rapidly-worsening heart and kidney function. Intra-aortic balloon pumps (IABPs) are commonly used but of minimal benefit in CHF. Hemodialysis and hemofiltration have been shown to be effective in removing extra fluid, reducing symptoms and improving heart function, but its use is limited to the Intensive Care Units (ICU) patient population.
Surgical solutions exist, but are only used for the treatment of very end-stage heart failure. These therapies (such as LVADs) are very effective at increasing blood flow. However, they are invasive, costly and require the patient to undergo heart transplantation. Even with the wide variety of existing therapies, over 2,300,000 CHF patients become hospitalized each year at a cost of over $10 billion dollars to the health care system. New CHF therapies are needed.
B. A Large Unmet Clinical Need in Patients with CHF for Enhanced Fluid Removal
If excessive fluid is not promptly removed with medication, CHF patients are often intubated and placed on a ventilator. If the initial diuretic therapy has little affect, more aggressive treatment with increasingly potent diuretics is needed. In addition, inotropic agents such as dobutamine are administered to increase the pumping function of the heart and rise the blood pressure. Higher blood pressure is expected to assist in the perfusion of the kidneys and make diuretics work. In more recent years vasodilator therapy became a part of the standard therapy for a severely volume-overloaded, decompensated CHF patient. All the above-mentioned therapies as a rule require admission to the ICU. Potentially dangerous side affects of drugs, needed for advanced monitoring and intubation, are the main reasons for a typical ICU admission.
While there are many potential factors that cause a patient to be hospitalized, the primary causes of admission in CHF patients are symptoms of severe shortness of breath from fluid overload. Standard drug therapy is unable to remove excess fluid rapidly enough to prevent hospitalization before any increased standard medical therapy has time to work. There is a clear and unmet clinical need for a CHF treatment that allows physicians to rapidly, controllably and safely remove a clinically significant amount of fluid from a CHF patient. Such a treatment would reduce the need for excessive hospital admissions.
Symptoms of fluid overload are excessive fluid retained in the abdomen, legs and lungs. Of these, fluid in the lungs is the most important. Patients have difficulty breathing. Edema in the lungs leads to poor blood oxygenation. Poor oxygenation leads to acidosis and deleterious neurological and hormonal phenomena that increases vasoconstriction and load on the heart. In addition, vasoconstriction leads to reduced blood flow to the kidneys and diminishes the effectiveness of the main pharmacological means of fluid removal—diuretic treatment. This phenomenon is known as the “vicious cycle” of CHF heart failure.
As previously mentioned, hemodialysis and hemofiltration can be used to remove excess fluid from a patient, especially in patients whose kidneys are not working. The term “Renal Replacement Therapy” generally refers to any forms of dialysis, solute and fluid balancing therapy. These treatments circumvent the kidney and replace kidney functions. These treatments are not generally applicable to CHF patients having functional kidneys, but which lack sufficient blood flow to properly perform their kidney functions, especially the removal of excess fluids, e.g., water, from the body.
1. Principles and Concept of Existing Methods of Renal Replacement Therapy
Renal replacement therapy performs two primary functions: ultrafiltration (removal of water from blood plasma), and solute clearance (removal of different molecular weight substances from blood plasma). The filter called “dialyzer” can be set up to perform either or both of these functions simultaneously, with or without fluid replacement, accounting for the various modes of renal replacement therapy. “Clearance” is the term used to describe the removal of substances, both normal and waste product, from the blood.
Ultrafiltration is the convective transfer of fluid out of the plasma compartment through pores in the membrane. The pores filter electrolytes and small and middle sized molecules (up to 20,000 to 30,000 daltons) from the blood plasma. The ultrafiltrate output from the filtration pores is similar to plasma, but without the plasma proteins or cellular components. Importantly, since the concentration of small solutes is the same in the ultrafiltrate as in the plasma, no clearance is obtained, but fluid volume is removed.
Dialysis is the diffusive transfer of small solutes out of a blood plasma compartment by diffusion across the membrane itself. It occurs as a result of a concentration gradient, with diffusion occurring from the compartment with higher concentration (typically the blood compartment) to the compartment with lower concentration (typically the dialysate compartment). Since the concentration of solutes in the plasma decreases, clearance is obtained, but fluid may not be removed. However, ultrafiltration can be combined with dialysis.
Hemofiltration is the combination of ultrafiltration, and fluid replacement typically in much larger volumes than needed for fluid control. The replacement fluid contains electrolytes, but not other small molecules. Since the net effect of replacing fluid without small solutes and ultrafiltration of fluid with small solutes results in net removal of small solutes, clearance is obtained.
While effective at removing excess fluid, substantial clinical data exists showing that ultrafiltration provides significant other benefits to patients with CHF. These benefits include the promotion a variety of compensatory neurohumoral mechanisms, such as activation of the renin-angiotensin-aldosterone system and stimulation of the sympathetic nervous system, resulting in both sodium accumulation and increased peripheral vascular resistance. Commonly, fluid removal with diuretics further enhances the neurohumoral stimulation and may even aggravate heart failure in some patients. Ultrafiltration interrupts this vicious cycle and represents an alternative approach to the treatment of refractory heart failure. Further beneficial effects of ultrafiltration include a subsequent increase in urine output, and an increased responsiveness to standard oral diuretic therapy.
For example, one study randomized congestive heart failure (NYHA Class II to III) to treatment with ultrafiltration (1.3 to 2.6 L over 3 to 5 hours) or with furosemide (potent diuretic). Both treatments produced similar hemodynamic and fluid losses. However, three months after intravenous furosemide treatment, hemodynamics and fluid volume had worsened back to baseline values, yet they were still significantly improved in the ultrafiltration group. The data suggest that fluid removal by ultrafiltration shifts the abnormal set point for fluid balance to a more physiologic level, an effect not accomplished by furosemide, despite comparable amounts of volume removal. Several other clinical studies showed similar beneficial results. Thus, ultrafiltration appears to be a beneficial in patients with CHF, even those still responsive to standard medical therapy.
2. Limitations of Existing Methods of Ultrafiltration to Treat CHF
Ultrafiltration has not been used widely in the treatment of patients with CHF, despite its apparent clinical benefits. There are several issues limiting the use of currently available ultrafiltration devices:
i. Prior ultrafiltration devices require central venous access (e.g., via surgery) with its attendant risk of infection, bleeding, collapsed lung and death.
ii. Prior ultrafiltration treatments require interaction and use by a nephrologist who is not the patient's primary physician, and who may be reluctant to expend their device and personnel resources on these patients.
iii. Prior ultrafiltration devices draw large blood volumes of blood out of the body and, thus, require central venous access. Moreover, the temporary large blood loss may lead to hypotension (low blood pressure) and the potential for large losses of blood.
iv. Prior ultrafiltration devices are generally designed to be only used in the ICU or dialysis unit environment.
v. Prior ultrafiltration devices require high blood volume flows to prevent clotting in the blood circuit and filter apparatus.
vi. Most patients are required to be anticoagulated leading to an increased risk of bleeding.
Continuous Veno Venous Hemofiltration (CVVH) allows removal of blood fluid and modification of the volume and composition of extracellular fluid to occur evenly over time. A filter that is highly permeable to water and small solutes but impermeable to plasma proteins and erythrocytes, is placed in the extracorporeal circuit. As the blood perfuses the hemofilter an ultrafiltrate is removed in a manner similar to glomerular filtration in the kidney.
Modern CVVH machines over time can provide almost complete renal replacement therapy (act as an artificial kidney) in an anuric patient. The technique is typically used in the ICU setting on a patient that has permanently or temporarily lost natural renal function as an alternative to intermittent dialysis. Secondary to the artificial kidney function, CVVH offers precision and stability that allows electrolytes or any appropriately sized element of circulation to be removed or added independently of changes in the volume of body water. In turn, if desired, the volume of water can be adjusted in a controlled fashion. Although valuable and powerful clinical tool, the versatility of CVVH limits its use in clinical practice and acceptance. The limitations of CVVH include:
i. Electrolyte removal and replacement is a high risk therapy. It is particularly risky in cardiac patients, since excess or depletion of electrolytes can cause arrhythmias. For this reason primarily, CVVH is prescribed and administered by a nephrologist.
ii. Large amounts of blood, in the range of 100-400 mL/min or as much as 10% of the total cardiac output for an adult patient, are passed through the filter. This necessitates the so-called “central” vascular access. Relatively-large and long catheters are threaded from a peripheral vein in an arm or a leg of the patient until they reach a large vascular volume in the center of the body where the sufficient blood flow is present. These cavities containing large volumes of blood can be the vena cava or the right atrium of the heart. To establish the central access a vascular surgeon or another similar specialist is required. Patients with central access catheters require additional monitoring. The central access is associated with serious complications.
iii. Tens of liters of fluid are continuously removed and replaced in a patient over the course of one day. If the desired balance is disrupted, patient can rapidly gain or lose fluid. The part of the machine responsible for fluid balance is called an ultrafiltration controller (UC). The UC of a modern CVVH machine has evolved into an extremely sophisticated apparatus capable of measuring the rate at which fluid is added or removed with the accuracy of less than 0.5%. This accuracy comes at a price of technical complexity and cost.
As a result, CVVH has in the past only been used in the ICU of a hospital where resources, training and adequate nursing monitoring are available. In addition, controls of CVVH machines are difficult to understand and require extensive training Although the latest and most sophisticated apparatus “Prisma” from Gambro takes advantage of interactive computer screens to simplify the task of setting up the device and controlling its use, it still requires many parameters to be configured before the device can be used.
Mechanical fluid removal such as SCUF (Slow Continuous Ultrafiltration) or CVVH is not used in these patients until it becomes obvious that the natural kidney function is insufficient. This typically is a result of an Acute Renal Failure (ARF) secondary to hypotension and hypoperfusion of the kidneys. The CVVH is prescribed by a nephrologist.
Abundant scientific and clinical evidence exists that aggressive early fluid removal with a machine would benefit CHF patients. It can reduce symptoms of fluid overload, prevent intubation, reduce load on the heart and reduce neuro-hormonal stimuli that drive vasoconstriction. To be truly affective the treatment shall precede the onset of ARF and shall be placed in the hands of a cardiologist who is the primary physician responsible for a CHF patient.
Current clinical use of CVVH in heart failure can be described as “too little too late”. As a result, CVVH is used for many days not as much for fluid removal but as an acute renal replacement therapy (similar to dialysis) in patients with lost renal function.
With the increasing prevalence of decompensated CHF and the increased cost of hospital admission and even more so of ICU treatment, a strong need has emerged for a new technology that will allow fluid removal in the non critical care setting. This need is for a device and technique that is simple and safe so that it could be used in the outpatient setting, doctors offices, Emergency Rooms (ER) and general hospital floors.